hmm... its kinda hard to do what you are asking to do.
sharing users, is not a problem, just copy users table, to another DB, and add the following code.
if you want them to share the same threads, then just open sub-forum for fishin and that will do.
what i suggest you to do, is open 2 main forums, and each one of them have Sub-forums, and you can play with them catagories and stuff like that.
then write some own made index page, that will direct users to both places.
this way, you'll be able to use one DB, sharing the users and not force them to sign up twice, and not getting into double postings.
you can also do diffrent styles for each one of them (vbulletin options), its a smart system, just know how to use it
hope you understood what i ment here, if not, let me know, ill re-explain myself for you.
greets,
Fury.